There are, and i had it listed in my original comment and then lost that part while editing. Thank you for calling it out! The cost is that we lose a fallback scenario where we construct a localization bundle out of resources mixed between packaged and language pack. Imagine a scenario where 20 resources are in langpack, and one is missing. Currently one of the fallbacks would be those 20 from langpack plus the remaining one from packaged. In the new model, we will load all 21 from packaged since we could find all 21 in langpack. (With a caveat that we also will gain flexibility in identifying how many missing resources invalidate a bundle, so maybe one missing file will not and then we will create a bundle with 20 locales from langpack, localize what we can, and then create a bundle with 21 from packaged and localize the rest). The bottom line is that we'll lose a "mixed" bundle with some resources from langpack and some from packaged. But we will not lose ability to end up with translations on a screen where what was available came from langpack, and the rest came from packaged, so i think the impact is acceptable. Lmk if you'd like me to document the trade-off better - it's a bit tricky to describe but i can try to draw some diagrams :)
Bug 1642415 Comment 63 Edit History
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There are, and i had it listed in my original comment and then lost that part while editing. Thank you for calling it out! The cost is that we lose a fallback scenario where we construct a localization bundle out of resources mixed between packaged and language pack. Imagine a scenario where 20 resources are in langpack, and one is missing. Currently one of the fallbacks would be those 20 from langpack plus the remaining one from packaged. In the new model, we will load all 21 from packaged since we could not find all 21 in langpack. (With a caveat that we also will gain flexibility in identifying how many missing resources invalidate a bundle, so maybe one missing file will not and then we will create a bundle with 20 locales from langpack, localize what we can, and then create a bundle with 21 from packaged and localize the rest). The bottom line is that we'll lose a "mixed" bundle with some resources from langpack and some from packaged. But we will not lose ability to end up with translations on a screen where what was available came from langpack, and the rest came from packaged, so i think the impact is acceptable. Lmk if you'd like me to document the trade-off better - it's a bit tricky to describe but i can try to draw some diagrams :)